1. Field of the Invention
The present invention relates to a pressure detecting apparatus of an injection molding machine, for detecting the pressure of a hydraulic cylinder such as an injection cylinder.
2. Description of the Related Art
The injection molding machine disclosed in the official gazette of Japanese Patent Laid-Open No. 1(1989)-263021 is generally known as an existing injection molding machine provided with a pressure detecting apparatus. The injection molding machine has an injection nozzle at its front end, a barrel cylinder into which a screw is inserted, and a pair of injection cylinders (hydraulic cylinders) having a built-in piston. Moreover, a rod protruding backward from the piston is connected with the rear end of the screw so that they can move forward or backward together.
The injection molding machine is also provided with a controller having a pressure detecting apparatus. The pressure detecting apparatus comprises a pressure sensor (hydraulic sensor) to detect the oil pressure of a rear oil chamber and an amplifier connected to the pressure sensor. Thereby, a detection signal of the pressure sensor is amplified by the amplifier to serve as a detected pressure value at the time of pressure control. The detected pressure value is compared with a set pressure value by a comparator and thereby the deviation between the detected pressure value and the set pressure value is obtained. The deviation is pressure-compensated by a pressure compensating section and moreover amplified by the amplifier, and thereafter outputted to a servo valve drive. A command signal is outputted from the servo valve drive to a servo valve, pressure-controlled oil is supplied to the injection cylinder, and pressure feedback control is performed so as to make the detected pressure value equal to the set pressure value.
Because the above existing pressure detecting apparatus frequently uses a strain gauge as a pressure sensor, the obtained detected pressure value includes errors due to the fluctuation of characteristics of the strain gauge and the fluctuation caused when setting the strain gauge to the injection cylinder. Therefore, a zero adjusting circuit and a gain adjusting circuit are normally built in the amplifier connected to the pressure sensor to perform zero adjustment and gain adjustment at the time of initial adjustment.
However, when temperature drift occurs due to various factors including an operating environment and an operating time zone of the injection molding machine, even if zero adjustment is performed at the time of initial adjustment, problems occur such that the adjustment point is deviated from the normal zero adjustment point and as a result, pressure is not accurately or precisely detected, involving deterioration of the quality of a molded part.